Late Term Abortion in Horse Pregnancies

Many horse owners breath a sigh of relief once their pregnant mare makes it through the third month of pregnancy, since that is when many pregnancies end in early abortion. However, horse owners and veterinarians should be aware of later threats to full term pregnancies in horses.

Mare caring for new foal

A number of conditions can befall late-term gestation in mares, resulting in abortion, but with swift and effective veterinary care your mare should be able to successfully carry a foal to term. New window.

A number of conditions can befall late-term gestation, seven months and beyond in mares, resulting in abortion. One of the most common causes of late-term abortion in horses is placentitis. An inflammation of the placenta often caused by an infection invading the uterus via the cervix, placentitis is responsible for up to 40% of late-term abortions in mares.

According to a report by C. Scott Bailey, DVM, an assistant professor of theriogenology at the North Carolina State University College of Veterinary Medicine, at the 2013 Society for Theriogenology Conference in Kentucky, 60% of placentitis cases are caused by bacterial infection, usually by Streptococcus zooepidemicus, or less often Leptospira spp. or E. coli. Fungal pathogens are also responsible for a percentage of cases.

Another cause of late term abortions is Equine herpesvirus. Ominously dubbed the “abortion virus,” a major cause of abortion in mares is equine herpes virus, most notably the EHV-1 and EHV-4 strains. If a pregnant mare becomes infected with EHV-1, or if she is carrying a latent infection that is activated by stress during pregnancy, the virus can cross the placenta and cause the foal to be aborted.

This usually occurs in late pregnancy, but can happen as early as the fourth month. Infection with EHV-1 or EHV-4 very late in pregnancy may result in a stillborn foal, or one that dies within a few days of birth.

Other conditions can also cause premature parturition or abortion, including torsion or strangulation of the umbilical cord, where excessive twisting or wrapping around a limb shuts off the flow of blood, resulting in the death of the fetus; twinning, which can be easily diagnosed and prevented with early ultrasound; and gastro-intestinal colic.

Before you as a horse owner lose sleep worrying about late-pregnancy complications, be assured that the vast majority of foals hit the ground safely without any help from humans at all.

And if the unthinkable happens and your mare develops complications during pregnancy, with swift and effective veterinary care your mare should be able to successfully carry a future foal to term.

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